Learn Swedish: Han råkade skriva en väldigt taktlös kommentar. - He accidentally wrote a very tactless comment.

Råka acts like an auxiliary or semi-modal verb here. Just like you wouldn't use att after kan (can) or vill (want to), you use the bare infinitive skriva directly after råkade to indicate an unintentional action.

Does råkade translate better to "accidentally" or "happened to"?

Both work perfectly! In Swedish, råka covers any unintentional or chance action. Here, "accidentally wrote" makes the most sense in English, but "happened to write" conveys the exact same grammar and underlying meaning.

Why does väldigt have a -t at the end if the noun kommentar is an en-word?

Because it is functioning as an adverb modifying the adjective taktlös, not the noun. When an adjective (like väldig) is used as an adverb (meaning "very"), it always takes the neuter -t ending, regardless of the gender of the noun being described.

What does the suffix -lös in taktlös mean?

Just like the English suffix "-less", -lös means "without". So taktlös literally means "without tact" (tactless). You will see this B2-level suffix in many other words, like arbetslös (unemployed) or hopplös (hopeless).